1. Field of the Invention
This invention relates to a developing device and an image forming apparatus which are particularly useful when used in a xerographic printing machine in which a latent image formed on an image supporting body is developed into a visible image as a toner is applied to the image supporting body over the latent image.
2. Description of Related Art
In a xerographic printing machine, a printing process begins with exposing an electrically charged photosensitive image supporting body (hereinafter also called the photosensitive drum) to light in a pattern of print to form a latent image on a circumferential surface of the photosensitive drum. In the meantime, a two-component developer in a mixture of toner and magnetic powder (carrier) is delivered to a circumferential surface of a developing roller to form a layer of the developer in a suitable thickness on the circumferential surface of the developing roller; the latent image formed on the photosensitive drum is developed into a visible image with the toner as the developing roller is rotated in the vicinity of the circumferential surface of the photosensitive drum.
Then the toner of the visible image on the circumferential surface of the photosensitive drum is transferred to a print sheet, whereupon the unfixed toner image on the print sheet is fixed to finalize the print process onto the print sheet.
In the developing device, a consumption of toner due to printing is calculated from the result of detection of a toner density sensor, a print density sensor, etc., and an amount of toner commensurate with the calculated consumption will be additionally supplied from a toner hopper into the developing device.
The developing device is provided with a plurality of stirring and conveying screws or puddles (hereinafter also called the screws) which are disposed downwardly of a toner delivery port of the toner hopper and contiguously parallel to one another. As they rotate about their respective axes, the screws convey the developer while stirring the developer to mix the toner and carrier with each other.
During the conveying and stirring, the screws are charged with electricity of a bias voltage, opposite in polarity to that applied to the photosensitive drum, by the power source so that the developer (toner) is charged with electricity via the screws.
In a developer (toner) delivery path to the photosensitive drum, the amount of electrical charge of the developer (toner) should usually exceed a predetermined value until it reaches the developing roller.
FIG. 14 is a graph showing an example of distribution of amount of electrical charge of a developer in the printing machine; the developer (00403/J5) shown in FIG. 14 was such that the most largest part of toner was charged in approximately 1.5 fC/.mu.m.
The developing roller includes a fixed magnet, in the form of an axle having a plurality of conveying magnetic poles, and a rotary sleeve, in the form of a hollow tube of non-magnetic material such as aluminum alloy, which is rotatably disposed around the fixed magnet; the sleeve is driven for rotation to convey a developer and also attracts the developer onto a circumferential surface of the sleeve while conveying.
Further, the developing roller is disposed in a parallel confronting relationship to the photosensitive drum with a small gap therebetween. At a position upstream of the developing region of the photosensitive drum, the developing roller receives the stirred and mixed developer from adjacent developer conveying rollers and the screws.
Thus the two-component developer is conveyed as the toner delivered from the toner hopper is stirred and mixed with the carrier by the stirring and conveying screws or puddles, during which the developer is charged up to a predetermined amount of electrical charge. The resulting developer is then transferred to the developing roller directly or via the developer conveying rollers.
The transferred developer on the developing roller is conveyed with rotation of the sleeve of the developing roller and is then restricted in amount (thickness of the developer layer) on the circumferential surface of the developing roller to a constant value by a restricting member. After that, only toner in the developer adheres to a latent image on the photosensitive drum to develop the latent image into a visible image.
In the meantime, the carrier in the developer keeps staying on the sleeve of the developing roller even after this developing on the photosensitive drum and is hence conveyed back into the developing device, whereupon the carrier is released from the developing roller, is then collected and is again stirred and mixed with fresh toner from the toner hopper by the screws.
In order to facilitate stirring and mixing toner with carrier by the screws, it has been customary to make toner particles round. And in order to prevent cohesion of toner, it has been also customary to add an additive to toner.
In the foregoing example, the two-component developer, which is composed of toner and carrier, was used. Alternatively a single-component developer, which is composed of only toner, may be used.
In recent years, as many of printing machines are often installed in offices rather than homes, demands for speeding and downsizing the printing machines are on the rise. Particularly in the above-mentioned xerographic printing machine, the developing process causing toner to stick to the photosensitive drum is important as being influential on improving the print quality and increasing the printing speed.
For achieving high-speed developing, it is necessary to carry out stirring and charging of toner at high speed; if the ability of stirring and charging toner is lowered with respect to printing speed, print faults, such as cohesion of toner particles and blurring would be encountered with printing, lowering the print quality.
Further, for increasing the printing speed, an improved developing device is known in which a developer is preliminarily stirred before being delivered to the stirring and conveying screws for further stirring. However, this developing device must be equipped with a mechanism dedicated for preliminary stirring, which would increase the entire device size.
Another improved developing device is known for crushing the cohered toner into small particles using a stirring member with projections. With this improved developing device, however, as the printing (developing) speed is increased, it would be impossible to electrically charge the toner sufficiently, thus resulting in occurrence of blurs in printing.